Quantum mechanics originally began at the turn of the ninteenth century. After trying to determine why hot objects glow the colour they do, Max Planck came up with a theory that energy was being emmited as light, in chuncks which he called "quanta".  In 1905 Albert Einstien confirmed this theory by showing that this is indeed what energy is made up of. Einstein termed these chunks 'photons'.

Though there was originally much controversy surrounding quantum mechanics, it did help solve many of the problems the physicists of that time had, concerning their observations of the natural world. One way quantum mechanics helped in solving a fundamental physcics problem was by changing the way we see the atom.

Back before quantum mechnics, there was a problem with the way we thought the atom behaved. In those times, there was only classical mechanics, which predicted that as the electron orbited the atoms nucleus, its orbit would eventually decay. But obviously this was not correct, because if it were, the atom would not be stable, and the atoms we see around us would probably not be here. 

In 1913, a solution was proposed by Physicist Neils Bohr. Bohrs solution was that the electron would orbit the nucleus at fixed energy levels. i.e. the further the electron was from the nucleus the lower its energy level was. With this theory, if  an atom emmited energy, then one of its electrons would shift energy levels. This fixed the problem of the decaying electron, but it also came with a paradox. Up to this point in history, it was known that light acted like a wave, but both Einstien and Planck theoried that light came in packets called photons. This would mean that light would have to act like both a particle and a wave. This created a big stur amongst physicsists.

Another interesting outcome which comes from Quantum Mechanics is that if you are not observing a particle, then it is in a state of 'superposition', which is just a fancy way of saying 'all of its possible positions', it is only when you observe the particle that it 'chooses' one of these possible locations.

According to Quantum Mechanics, everything acts as both a wave and a particle at once. Though this goes against common sense, it has become a cornerstone of modern physics, and continues to be added upon to this day.